It is often useful to combine a plurality of optical image projectors to produce a composite image. Such a combination might be useful where an image brighter than the capabilities of a single projector is desired. A color image may be produced using three primary color projectors. A two projector system might be used to produce a stereo picture display. In any of these applications it is important that each projector be made to project an image that is in registry with the images of all of the other projectors in the display.
In scanning type projectors it is customary to modulate the image deflection signals with signals called convergence signals. In a scanner the deflection signals are usually arranged to provide as linear an image as possible. Then an analog correction signal is added to the scanning signal. This analog signal is shaped in a so-called convergence procedure so that the projectors produce convergent pictures. Typically the prior art convergence signals were such that adjusting convergence at one part of the picture would affect convergence in another part so that the convergence process included a very time consuming repetitive adjustment or tuning sequence.
Desirably convergence would be such that once an area of a picture is converged it need not be touched again as other areas are converged. Thus the convergence process needs to be performed only once and the initial adjustment can be made highly precise.